1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
An image forming apparatus, such as a laser beam printer, a digital copying machine, or a laser facsimile, includes an optical scanning device for forming a latent image on a surface of an image carrier (photosensitive element). Some optical scanning devices are configured such that a light flux (light beam) emitted from a light source is deflected by a rotating deflector whereby a surface of a photosensitive element is irradiated with the light flux.
In recent years, an image forming apparatus is required to achieve a higher print speed and a higher pixel density, and to satisfy such a requirement, the image forming apparatus needs to include a polygon scanner capable of rotating at a high speed from 40,000 revolutions per minute (rpm) to 50,000 rpm. Major technical problems in manufacturing the polygon scanner capable of rotating at the high speed are vibration noise generated by a motor and an increasing amount of generated heat in accordance with increase in power consumption due to the high speed rotation of the polygon scanner.
To solve the problem about noise, the image forming apparatus needs to be designed to achieve a quiet operation as well as the higher print speed and the higher pixel density. Because attention is particularly focused on noise generated by the polygon scanner as a problem about noise in performance of the image forming apparatus, it is important that a system needs to be designed to achieve a quiet operation.
The vibration noise generated by the motor is classified into four types of noises, i.e., wind roar caused by a polygon mirror, vibration noise caused due to bending of a rotary shaft, noise caused due to excitation of a wound coil of the motor, and vibration noise caused due to friction of a bearing. Theses noises are transmitted to outside as vibration noises, resulting in leakage of noise.
Furthermore, a higher current value is required to achieve the high speed rotation of the polygon scanner, and therefore temperature inside an optical scanning device (writing unit) is increased due to an increasing amount of generated heat in accordance with increase in power consumption. If a laser beam emitted from a laser included in the optical scanning device is interrupted by dust along a scanning line of the laser beam, an image is printed out with a white line formed thereon. To prevent dust inside the optical scanning device, the optical scanning device needs to be completely sealed.
Because a large number of components of a scanning optical system, such as a lens that is easily affected by heat, are mounted in the optical scanning device, if the polygon scanner that is a heat source is mounted in a sealed housing, the optical scanning device needs to have a configuration for efficiently exhausting heat. According to a specification of a polygon scanner capable of rotating at more than 40,000 rpm as recently required, it is difficult for the polygon scanner itself to achieve the reduction of noise and the efficient heat exhaust.
Japanese Patent Application Laid-open No. 2006-35570 discloses an image forming apparatus in which a polygon-scanner attachment member is arranged as a separate member to attach a polygon scanner to an optical housing on which a component of a scanning optical system is mounted, and the optical housing and the polygon-scanner attachment member are fixed to a main-body frame of the image forming apparatus.
Japanese Patent Application Laid-open No. H8-2000 discloses an image forming apparatus in which a component of a scanning optical system is attached to an optical housing and a polygon scanner is fixed to a main-body frame of the image forming apparatus, so that vibration of the polygon scanner is not directly transmitted to the scanning optical system.
Japanese Patent Application Laid-open No. 2001-296495 discloses a configuration for sealing a polygon scanner by a heat releasing member whereby heat release and noise reduction are achieved.
Japanese Patent No. 2774444 discloses a stand-alone configuration of a polygon scanner in which an area where a polygon mirror is mounted is completely sealed so that leakage of noise is prevented.
Japanese Patent Application Laid-open No. 2002-107649 discloses a stand-alone configuration of a polygon scanner in which an elastic sealing member is arranged to reduce leakage of wind roar caused by a polygon mirror.
However, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-35570, because the polygon-scanner attachment member is directly fixed to the main-body frame, tilt characteristics of a surface of a polygon mirror depends on assembly accuracy of the main-body frame, which can cause pitch irregularity in a sub-scanning direction. Furthermore, if the polygon scanner is tilted, a laser light can be projected outside of an effective area of the polygon mirror. Thus, it is difficult to achieve mass production.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. H8-2000, because the polygon scanner is directly fixed to the main-body frame, problems similar to those in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-35570 can occur.
In the configuration disclosed in Japanese Patent Application Laid-open No. 2001-296495, it is necessary to obtain surface accuracy of a surface of the heat releasing member to be in contact with an optical housing and surface accuracy of an attachment surface of a polygon scanner board because of tilt characteristics of a surface of a polygon mirror. Therefore, it is difficult to use a heat releasing member manufactured by pressing. Furthermore, resonance with the heat releasing member can occur. Moreover, because the polygon scanner that is a heat source is completely sealed in a small space, it is difficult to release heat.
In the configuration disclosed in Japanese Patent publication No. 2774444, because a polygon-scanner board is fixed to a polygon housing and the polygon housing is attached to an optical housing, it is difficult to achieve efficient heat exhaust for the high speed rotation of the polygon scanner. This is because heat exhaust efficiency is improved if a component that is resistant to heat is not mounted in a heat exhaust pathway.
In the configuration disclosed in Japanese Patent Application Laid-open No. 2002-107649, it is difficult to perform efficient heat exhaust, and the noise reduction and the prevention of heat generation cannot be achieved in the polygon scanner capable of rotating at a high speed.